SafetyCritical Systems 2

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Safety-Critical Systems 2

• T 79.232
• Risk analysis and design for safety
• Ilkka Herttua

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V - Lifecycle model
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Overall safety lifecycle

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Risk Analysis

• Risk is a combination of the severity (class) and
  frequency (probability) of the hazardous event.
• Risk Analysis is a process of evaluating the
  probability of hazardous events.
• The Value of life??
• Value of life is estimated between 0.75M 2M
  GBP.
• USA numbers higher.

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Risk Analysis

• Classes - Catastrophic multiple deaths
  gt10
• - Critical a death or severe injuries
• - Marginal a severe injury
• - Insignificant a minor
  injury
• Frequency Categories
• Frequent 0,1 events/year
• Probable 0,01
• Occasional 0,001
• Remote 0,0001
• Improbable 0,00001
• Incredible 0,000001

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Hazard Analysis

• A Hazard is situation in which there is actual or
  potential danger to people or to environment.
• Analytical techniques
• - Failure modes and effects analysis (FMEA)
• - Failure modes, effects and criticality
  analysis (FMECA)
• - Hazard and operability studies (HAZOP)
• - Event tree analysis (ETA)
• - Fault tree analysis (FTA)

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Fault Tree Analysis 1
The diagram shows a heater controller for a tank
of toxic liquid. The computer controls the heater
using a power switch on the basis of information
obtained from a temperature sensor. The sensor is
connected to the computer via an electronic
interface that supplies a binary signal
indicating when the liquid is up to its required
temperature. The top event of the fault tree is
the liquid being heated above its required
temperature.
 
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Fault event not fully traced to its source
Basic event, input
Fault event resulting from other events
OR connection
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Risk acceptability

• National/international decision level of an
  acceptable loss (ethical, political and
  economical)
• Risk Analysis Evaluation
• ALARP as low as reasonable practical (UK, USA)
• Societal risk has to be examined when there is
  a possibility of a catastrophe involving a large
  number of casualties
• GAMAB Globalement Au Moins Aussi Bon not
  greater than before (France)
• All new systems must offer a level of risk
  globally at least as good as the one offered by
  any equivalent existing system
• MEM minimum endogenous mortality
• Hazard due to a new system would not
  significantly augment the figure of the minimum
  endogenous mortality for an individual
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Risk acceptability

• Tolerable hazard rate (THR) A hazard rate which
  guarantees that the resulting risk does not
  exceed a target individual risk
• SIL 4 10-9 lt THR lt 10-8 per hour
  and per function
• SIL 3 10-8 lt THR lt 10-7
• SIL 2 10-7 lt THR lt 10-6
• SIL 1 10-6 lt THR lt 10-5
• Potential Loss of Life (PLL) expected number of
  casualties per year
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Current situation / critical systems

• Based on the data on recent failures of critical
  systems, the following can be concluded
• Failures become more and more distributed and
  often nation-wide (e.g. commercial systems like
  credit card denial of authorisation)
• The source of failure is more rarely in hardware
  (physical faults), and more frequently in system
  design or end-user operation / interaction
  (software).
• The harm caused by failures is mostly economical,
  but sometimes health and safety concerns are also
  involved.
• Failures can impact many different aspects of
  dependability (dependability ability to deliver
  service that can justifiably be trusted).

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Examples of computer failures in critical systems
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Driving force federation

• Safety-related systems have traditionally been
  based on the idea of federation. This means, a
  failure of any equipment should be confined, and
  should not cause the collapse of the entire
  system.
• When computers were introduced to safety-critical
  systems, the principle of federation was in most
  cases kept in force.
• Applying federation means that Boeing 757 / 767
  flight management control system has 80 distinct
  microprocessors (300, if redundancy is taken into
  account). Although having this number of
  microprocessors is no longer too expensive, there
  are other problems caused by the principle of
  federation.

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Designing for Safety

• Faults groups
• - requirement/specification errors
• - random component failures
• - systematic faults in design (software)
• Approaches to tackle problems
• - right system architecture (fault-tolerant)
• - reliability engineering (component, system)
• - quality management (designing and producing
  processes)

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Designing for Safety

• Hierarchical design
• - simple modules, encapsulated functionality
• - separated safety kernel safety critical
  functions
• Maintainability
• - preventative versa corrective maintenance
• - scheduled maintenance routines for whole
  lifecycle
• - easy to find faults and repair short MTTR
  mean time to repair
• Human error
• - Proper HMI

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Hardware Faults

• Intermittent faults
• Fault occurs and recurrs over time (loose
  connector)
• Transient faults
• Fault occurs and may not recurr (lightning)
• Electromagnetic interference
• Permanent faults
• Fault persists / physical processor failure
  (design fault over current)

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Fault Tolerance

• Fault tolerance hardware- Achieved mainly by
  redundancy Redundancy- Adds cost, weight, power
  consumption, complexityOther means- Improved
  maintenance, single system with better materials
  (higher MTBF)

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Redundancy types

• Active Redundancy
• Redundant units are always operating.
• Dynamic Redundancy (standby)
• Failure has to be detected
• Changeover to other modul

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Hardware redundancy techniques

• Active techniques
• Parallel (k of N)
• Voting (majority/simple)
• Standby
• Operating - hot stand by
• Non-operating cold stand by

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Reliability prediction

• Electronic Component
• Based on propability and statictical
• MIL-Handbook 217 experimental data on actual
  device behaviour
• Manufacture information and allocated circuit
  types
• Bath tube curve burn in useful life wear out

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Safety-Critical Hardware

• Fault Detection
• Routines to check that hardware works
• Signal comparisons
• Information redundancy parity check etc..
• Watchdog timers
• Bus monitoring check that processor alive
• Power monitoring

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Safety-Critical Hardware

• Possible hardware
• COTS Microprocessors
• - No safety firmware, least assurance
• Redundancy makes better, but common failures
  possible
• Fabrication failures, microcode and
  documentation errors
• Use components which have history and
  statistics.

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Safety-Critical Hardware

• Special Microprocessors
• Collins Avionics/Rockwell AAMP2
• Used in Boeing 747-400 (30 pieces)
• High cost bench testing, documentation, formal
  verification
• Other models SparcV7, TSC695E, ERC32 (ESA
  radiation-tolerant), 68HC908GP32 (airbag)

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Safety-Critical Hardware

• Programmable Logic Controllers PLC
• Contains power supply, interface and one or more
  processors.
• Designed for high MTBFs
• Firmware
• Programm stored in EEPROMS
• Programmed with ladder or function block
  diagrams

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Safety management

• Safety culture/policy of the organisation
• - Task for management ( Targets )
• Safety planning
• - Task for safety manager ( How to )
• Safety reporting
• - All personal
• - Safety log / validation reports

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Home assignments

• 4.18 (tolerable risk)
• 5.10 (incompleteness within specification)
• Email before 2. March to herttua_at_eurolock.org